1. Field of the Invention
The present invention relates generally to spark ignition devices, such as spark plugs, and more particularly to spark plugs having an integrated combustion sensor.
2. Related Art
Spark plugs have been used for many years to provide a means to ignite the fuel air mixture in the combustion chambers of spark ignition internal combustion engines. Spark plugs have taken on many forms to adapt to the particular engine design and environment. Generally, spark plugs have a center electrode surrounded by an insulator wherein the insulator is disposed in and captured by a metal housing or shell. The shell typically has a plurality of threads which are matched to the threads in the cylinder head in a hole called spark plug hole. The threads allow the spark plug to be screwed into the cylinder head using a conventional tool. Further, the shell includes at least one ground electrode which is either located on or extending from an end of the shell proximate the center electrode. The ground electrode together with the center electrode define a spark gap. The shell also acts as a ground shield to provide an electrical ground path from the spark gap to the engine block. The spark plug seats or seals against the engine cylinder head to seal the combustion chamber and prevent combustion gases from escaping through the spark plug hole in the cylinder head.
There are trends in spark ignition engines toward ever greater power output and efficiency, as well as toward the use of flexible fuel types, which together have increased the demand for and usage of various types of combustion sensors necessary to achieve these objectives by enabling enhanced control of the engine and combustion processes.
Combustion sensors, particularly combustion pressure sensors, have generally been discrete sensors that are inserted into the combustion chamber through special threaded bores created just to accommodate these sensors, and the sensors themselves have generally been used only in engine and engine control development and not in mass production owing to their high cost and the additional demands that their use places on space in and adjacent to the cylinder head. Increasingly, engine designs employing multiple valves, multiple fuel injection points, coil on plug ignition systems, other combustion-related sensors and other features have placed increasing demands on the space envelope in and adjacent to the cylinder head immediately adjacent to the combustion chamber, particularly the space above the combustion chamber, which have in turn made it desirable to reduce the total space envelope needed for spark plugs and combustion pressure sensors. Further, use of combustion pressure sensors in conjunction with mass production engines and engine controllers necessitates the design of sensors having a substantially reduced cost compared to these discrete pressure sensors.
In this regard, combination of a spark plug and a piezoelectric combustion sensor has been proposed in U.S. Pat. No. 6,756,722. In the '722 patent, a spark plug having a metallic shell with an annular central bore, a ceramic insulator also having a center bore which is fixed to the shell by deforming a portion of the shell and also retained by a formed flange within the annular bore of the shell, a center electrode located within the center bore of the insulator and a ground electrode attached to the shell and spaced from the center electrode to form a spark gap. The spark plug of the '722 patent is equipped with a cylindrical piezoelectric sensor formed from a number of cylindrical components which are located on the external surface of the spark plug. The piezoelectric sensor components are captured between a metallic holder which also includes on one end a hex head for attaching the sparkplug and a flanged cylindrical seat which adapted to seat against one of the insulator or turn-over of the shell. A pair of cylindrical washers are placed proximate to and between the holder and seat to provide respective bearing surfaces and protect a respective pair of piezoelectric ceramic elements which are separated from one another by a single cylindrical electrode. An insulator is placed on the seat proximate the piezoelectric ceramics and the electrode to electrically isolate them from the seat. An o-ring seal is placed in a groove located in the inner bore of the seat to provide a water-tight seal between the seat and the spark plug insulator. During manufacture and assembly of the device, the sensor components are placed over the insulator and shell of the assembled spark plug and compressively preloaded between the holder and the seat. Once the predetermined preload is achieved, the holder is fixed to the shell by laser-welding to complete the assembly of the spark plug and sensor. During operation of the spark plug having the integral pressure sensor shown in the '722 patent in an internal combustion engine, with each combustion of the fuel-air mixture the pressure of the expanding combustion gases presses the insulator, shell, or both of them, depending on the particular configuration of the sensor and spark plug, against the seat, thereby further compressing and loading the piezoelectric elements and producing an electrical output signal which is related to the pressure exerted by the combustion gases. Hence, the device of the '722 patent is adapted to both provide the spark for spark ignition and an output signal indicative of the resultant combustion pressure.
Another spark plug with an integral pressure sensor has been described in conjunction German Utility Patent Application DE 102005062881. The '881 application describes a spark plug having similar spark plug elements as those described above with regard to the '722 patent, such that they are not repeated herein for brevity. However, the construction of the pressure sensor is somewhat different from that described in the '722 patent. Whereas the '722 patent described a sensor assembly on the exterior of the spark plug, particularly the shell and the insulator, the '881 application describes a piezoelectric pressure sensor that is located on the interior of the spark plug, particularly between a portion of the insulator and the shell. In the '881 publication, the insulator is supported within the shell on a shoulder that corresponds with the core nose region of the insulator. The insulator and shell also each have additional lower shoulders that together form a parallelogram-shaped cavity that is used to capture the elements of the piezoelectric sensor, which include a tapered piezoelectric cylinder which is tapered so as to mate with the shoulder of the insulator and a tapered spring washer that is tapered so as to mate with the shoulder of the shell. A circular electrode is formed on the surface of the lower shoulder of the insulator and provides an electrical contact for the tapered surface of the piezoelectric element. The circular electrode is electrically connected to a vertical electrode section which is in turn electrically connected to another circular electrode formed on the free surface of the insulator which is adapted to permit external electrical connection to and output from the sensor during operation of the pressure sensor. During assembly of the spark plug, the insulator, shell and piezoelectric sensor assembly are given a compressive preload in conjunction with forming an upper shoulder of the shell. During operation of the '881 spark plug having the integral pressure sensor in an internal combustion engine, with each combustion of the fuel-air mixture the pressure of the expanding combustion gases tends to elastically press the insulator away from the shell in the space which houses the sensor assembly, thereby cyclically reducing the assembly preload and producing an electrical signal which is related to the pressure exerted by the combustion gases.
PCT patent application WO-2008/003846 and illustrated by FIG. 9 discloses yet another configuration of a spark plug having an integral pressure sensor. In this application, the outer surface of the shell 208 has a threaded portion 220 which is used for threading the spark plug into a cylinder bore. Above the threaded portion 220 is a thread undercut 222 which transitions between threaded zone 220 and the barrel. The thread undercut 222 transitions to a gasket flange 224 of the barrel which is wider than the remainder of the barrel and narrows via a shoulder to the upper portion of the barrel. The upper portion of the barrel of the shell has a generally uniform wall thickness with a small deformable area or buckle zone of reduced wall thickness (prior to assembly) which is radially inwardly and axially collapsed in conjunction with heating of this portion during assembly of the spark plug to form a gas-tight seal and mechanically fix (i.e., “hot lock”) the shell to the insulator. Following assembly, the upper portion of the barrel has a substantially uniform wall thickness. At the free end of the upper portion of the barrel opposite the gasket flange, a turn-over or flange 218 exists which includes shoulder 216 which is also formed in conjunction with spark plug assembly and captures the insulator 206 within the shell 208. The piezoelectric sensor assembly 230 abuts the lower shoulder and extends along the upper portion of the barrel. The piezo sensor assembly 230, may include either a piezoelectric element or a piezoresistive sensor element, and includes an intermediate or lower bushing 226 which is formed from a metal, such as steel, which extends in generally L-shaped in cross-section from a narrower section which abuts the barrel shoulder and buckle zone to a broader section which provides a seat for the lower ring electrode 234 on its lower surface. The upper surface of lower ring electrode 234 provides a mechanical seat and electrical contact for piezo element 232. Piezoelectric element 232 is in the form of a cylindrical ring or disk having a rectangular cross-section. Piezo element 232 may be either a piezoelectric element or a piezoresistive element. The lower surface of upper ring electrode 234 also provides an opposing mechanical seat and electrical contact for piezoelectric element 232. The upper surface of upper ring electrode 234 seats against the lower surface of insulator ring or disk 236. Insulator disk 236 abuts and electrically isolates the upper ring electrode 234 from the end of upper bushing 238. The inner diameter of the upper ring electrode 236 and the portion of outer diameter of the barrel to which it is adjacent are selected to provide a spacing sufficient to provide electrical isolation of the upper ring electrode 236 from the barrel. Upper bushing 238 engages and is welded to the outer surface of the barrel. The elements of the piezo sensor assembly 230 are in touching contact under a compressive preload by pressure applied from upper bushing 238 to the upper surface of the insulator disk 236. The welding of the upper bushing 238 to the barrel fixes the preload. The upper ring electrode 234 also includes an axially extending terminal connection 240 for electrical connection to a signal line for transmitting the signal output from the piezo sensor assembly 230. During operation of the '846 spark plug having the integral pressure sensor in an internal combustion engine, with each combustion of the fuel-air mixture the pressure of the expanding combustion gases tends to axially press outwardly against the insulator 206 from the sparking end associated with center electrode 202, ground electrode 204 and the spark gap formed thereby. These cause the insulator 206 to bear against the shell in the turn-over 218 and cause cyclic elastic tensile deformation of the shell 208 in the region between the turn-over 218 and the gasket flange of the barrel 224 which is also the region of the shell 208 which is proximate the piezo sensor assembly 230. This tensile stretching of the shell 208 also cyclically reduces the assembly preload of sensor 230 and produces an electrical signal which is related to the pressure exerted by the combustion gases and may be output through terminal 240. The sensor assembly also includes a cover plate 242 to shield the other sensor elements from mechanical damage, the ingress of dirt, water or other contaminants and to suppress the effects of radio frequency interference.
While the device of the '846 application has some similarity to the '722 patent in that the elements of the sensor are located on the exterior of the spark plug insulator and shell, it is distinguished from the '722 patent by the nature and arrangement of the sensor elements, as well as the means by which it interacts with the insulator and shell during operation of the device. For example, the '846 patent application describes a single piezoelectric element, as contrasted with two in the '722 patent, and it does not include an electrode as does the sensor assembly described in the '722 patent. Further, the piezoelectric sensor of the '085 patent application is attached only to the shell and is borne on by the shell, as contrasted with the various embodiments of the '722 patent in which the sensor is attached to the shell and borne on by a combination of the insulator and cover, or the insulator and shell and cover. The '846 application is similar to the '881 patent in that the sensor is compressively preloaded and during operation of the spark plug is cyclically unloaded as the insulator is pressed axially outwardly against the shell, thereby cyclically unloading a portion of the preload pressure as a result. However, the internal construction of the pressure sensor and use of differing elements and their arrangement distinguish the spark plug of the '085 application from that of the '881 application.
While such prior art spark plug designs having integrated pressure sensors each differ from one another, they represent examples of the progress in the art. However, there remains a need for integrated spark plugs with integrated combustion gas sensors which further improve the progress of the art.